Field of the Invention
The present invention relates to a method for segmenting the surface of a tyre and to an equipment operating according to this method. The method and equipment according to the invention can be used to detect in a precise and accurate way areas on the tyre surface having different characteristics, and consequently to segment the surface of the tyre into groups of areas or pixels having similar properties.
Description of the Related Art
Tyres, particularly top-range models, but also those not rated as high-performance, are usually carefully inspected after building and/or after vulcanization, in order to measure or discover a plurality of characteristics of the tyres, the values of which may, for example, lead to the rejection or acceptance of the tyres. For example, the tyre surface is carefully examined in order to discover any defects, non-uniformities or other faults, and the tyre is considered to be acceptable if these measured parameters fall within certain ranges of acceptable values, or if they are found to be absent or present. These ranges of values vary with the type, model, size and intended use of the tyres.
A tyre generally comprises a carcass structure, in the shape of a toroidal ring, including one or more carcass plies, reinforced with reinforcing cords lying in radial planes (in the case of what are known as radial tyres), in other words in planes containing the axis of rotation of the tyre. The ends of each carcass ply are fixed to at least one metal annular structure, usually known as the bead core, which reinforces the beads, in other words the radially inner extremities of said tyre, which serve to fit the tyre on a corresponding mounting rim. A band of elastomeric material, called the tread band, is placed on the crown of said carcass structure, and a relief design for ground contact is formed in this tread band at the end of the curing and moulding stages. A reinforcing structure, usually known as the belt structure, is placed between the carcass structure and the tread band. In the case of tyres for a car, this belt structure usually comprises at least two radially superimposed strips of rubberized fabric provided with reinforcing cords, usually metallic, positioned parallel to each other in each strip and crossing the cords of the adjacent strip, the cords preferably being positioned symmetrically about the equatorial plane of the tyre. Preferably, said belt structure also comprises a third layer of fabric or metal cords placed circumferentially (at 0 degrees) in a radially outer position, at least on the ends of the underlying strips.
Sidewalls of elastomeric material are also applied to the corresponding lateral surfaces of the carcass structure, each extending from one of the lateral edges of the tread band to the position of the corresponding fixing annular structure to the beads.
The term “groove” (in a tyre) denotes a recess formed on one of the surfaces of the tyre, not necessarily in the surface of the tread, but also, for example, in the radially inner surface of the tyre. In the specific case of the tread surface, a groove is defined as a recess forming part of the tread design defined above, which preferably separates two blocks of the tread. Since the groove is a recess, it is possible to identify a bottom surface and a top surface, where the top surface essentially coincides with the radially outer surface of the blocks separated by the groove, and the bottom surface is defined as a portion of the tyre surface spaced apart radially from the top surface and in a radially inner position with respect to the latter. The bottom and top surface are preferably continuously interconnected by lateral walls, which may be essentially perpendicular to a plane locally tangent to the top surface, or inclined with respect thereto. The bottom surface and lateral walls are considered to belong to the groove, while the top surfaces are considered not to belong to the groove. In the tyre surface, therefore, it is possible to define portions of surface belonging to a groove, in other words portions of surface belonging to either the bottom surface or the lateral walls, and portions of surface not belonging to a groove, in other words those forming part of the top surface.
Generally, a groove extends in a direction called the longitudinal direction, and in particular it has a longitudinal extension greater than its transverse dimension, although other geometries may be encountered.
A “defect” in a tyre is a characteristic of the tyre which is undesired, although it does not necessarily lead to the rejection of the tyre. Examples of types of defect are foreign bodies in the covering, under-vulcanization, open joints, defects within the grooves such as bubbles under a tread block, flash and steps on the tread, and exposed cords on the tread.
The term “exposed cord” denotes a tyre defect in which a textile or metal cord, for example (but not necessarily) one placed at 0 degrees, is “exposed” by the elastomeric material, or is at least visible under the material, in a groove of the tyre, because the elastomeric material is too thin. The elastomeric material above the cord has a “corded” appearance (hence the name) which is particularly difficult to identify.
The term “source of electromagnetic radiation” denotes one or more of a variety of radiation sources, including, but not limited to, sources based on LEDs (using one or more LEDs as defined below), incandescent sources (for example, filament lamps or halogen lamps), fluorescent sources, phosphorescent sources, discharge sources, lasers, and others.
A radiation source may be configured to generate electromagnetic radiation within the visible spectrum, outside the visible spectrum, or in both of these. The expression “source of light radiation emitting radiation in the visible spectrum” therefore signifies a source that emits radiation either in the visible spectrum only, or in the visible spectrum and in other spectra also. Since we are specifically concerned with radiation sources in the visible spectrum in the present context, the terms “radiation” and “light” are used interchangeably. Furthermore, a radiation source may include one or more filters, lenses or other optical components.
The expression “telecentric” radiation signifies a radiation source that emits electromagnetic radiation in the form of a plurality of rays essentially parallel to each other.
The expression “almost telecentric” radiation signifies a radiation source that emits electromagnetic radiation in the form of a plurality of rays forming an angle of less than about ±10° with each other. Consequently, “almost telecentric” sources include telecentric sources. Radiation is also described as essentially grazing with respect to a surface when the rays making up the radiation form an angle of not more than about ±15°, or more preferably about ±5°, with the surface.
The term “spectrum” is to be interpreted as referring to one or more radiation frequencies produced by a radiation source. The term “visible spectrum” generally denotes light radiation having a wavelength in the range from about 380 nm to about 760 nm.
The term “colour” of a radiation is used here interchangeably with the term “spectrum”. However, the term “colour” is used primarily to refer to a property of the radiation that can be perceived by an observer.
In the following text, the term “LED” refers to light-emitting diodes of any type which are configured to emit radiation in a specified spectrum. An LED therefore includes, but is not limited to, a semiconductor structure which emits radiation in response to a current. Organic semiconductor structures (for example OLEDs) are also included in the present definition. The term “different colour LEDs” therefore denotes LEDs emitting radiation with separate spectra, in other words spectra having different bandwidths and/or spectral components. The term “colour” generally refers to radiation in the visible spectrum, but the radiation (or “light”, interpreted as having the same meaning) emitted by the LEDs may also be within the infrared range, the ultraviolet range or a combination thereof with visible light.
The description of a radiation source, for example an LED, as “white”, “red”, etc., is to be interpreted as meaning a source emitting radiation predominantly perceived as “white”, “red” and so on, although the source may also emit other radiation.
US 2010/0002244 describes a method for inspecting the surface of a tyre capable of reliably discriminating rubber pieces having a quality different from that of the tyre, these pieces being incorporated in the tyre surface as a result of the vulcanization of the tyre. A first illumination unit includes two light projectors which project light from two opposite sides towards an objective line on a tyre. A second illumination unit includes a pair of second projectors which project light from opposite sides towards the objective line in a different direction from that in which the first illumination unit projects light. The first and second units operate alternately. A linear video camera forms an image of part of the tyre surface corresponding to the objective line in synchronization with the illumination process of the first and second illumination units. The images are analysed for the inspection of the tyre surface.
US 2011/0018999 describes a device for evaluating the appearance of a tyre surface, comprising a colour linear video camera including means for separating a light beam reflected by the surface of said tyre and entering the video camera in at least two primary colours having given wavelengths, such that the light beam is directed towards a number of sensors such that a basic grey scale image is obtained for each primary colour. The device also includes a number of lighting means equal to the number of the primary colours, said lighting means being orientated so as to light the surface to be evaluated at different angles. The device is characterized by the fact that each lighting means emits a coloured light which is different from that emitted by the other lighting means, and whose wavelength essentially corresponds to the wavelength of one of the primary colours selected by the video camera.
WO 2012/052301 describes a method for inspecting a tyre tread having a tread design formed by the set of circumferentially juxtaposed elements separated by boundaries having known identical shapes and a reduced number of basic patterns placed in sequence in a predetermined way.